How to add business unit name to invoice approval notification

Summary:

I have been trying to add the business unit name to the Invoice Approval Notification in BPM Worklist but the notification shows as "NULL" where the Business Unit name should show up.

Content (please ensure you mask any confidential information):

For my Invoice Approval Workflow notification, I have inputted "FAI Approval of {0} Invoice {1} for Amount {2} from Supplier {3} from Business Unit {4} " as my Argument, in which the expression that I have entered for Business unit is : /task:task/task:payload/ns2:findInvoiceHeader1Response/ns2:result/ns1:Value/ns1:BuName" that i have found in the drop down list of task:payload. However, the notifications' elements are all correct except for Business Unit that show up as "NULL" instead of writing the name of the Business Unit. Does anyone know how to fix this ?